Fix braces style
Sort methods
Refactor index checking on abc::str_base_ and abc::vector_base operator[]

Consolidate the code from these two implementations into the new
_raw_vextr_impl_base::validate_index().

Also change the type of the argument of operator[] to intptr_t, to check
for negative indices in a clearer way than expecting the sign bit turned
into MSB to have made the index too big. You didn’t get that? Exactly.